The present invention relates to a frame structure for a vehicle equipped with a pipe-shaped frame body which is provided as part of a vehicle body and receives a load acting from the outside so as to have bending deformation at a vehicle collision, for example.
Many frame structures which can provide a properly high bending rigidity and surely protect passengers in the vehicle in case of the load acting at the vehicle collision have been proposed. Japanese Patent Laid-Open Publication No. 2009-113766, for example, discloses a structure in which a reinforcement to form plural closed cross sections together with an inner panel of a pipe-shaped side sill is arranged at a corner portion of a vehicle body. This reinforcement has concave and convex portions (uneven) in a sectional direction of the side sill which is perpendicular to a longitudinal direction of the side sill. Herein, the convex portions are joined to an inner face of the corner portion. The above-described publication discloses results of comparison of a side sill equipped with the above-described reinforcement and a side sill without this reinforcement, by analyzing bending resistance characteristics of these side sills in case they have bending deformation. It also shows that the side sill equipped with the reinforcement can perform the higher bending resistance based on the comparison results.
As shown in the above-described publication, the bending deformation in case of the collision load acting can be restrained effectively without using any thick reinforcing member, by arranging the above-described reinforcement at the corner portion of the frame. Herein, the value of the bending resistance divided by the entire weight of the frame, that is, the so-called performance-weight efficiency, can be increased, so that the frame can be made properly light. The fuel economy (gas mileage) has been recently desired from an environmental protection perspective, and therefore various researches for the light-weight vehicle body have been conducted. For this reason, frames for vehicles which can have the lighter weight but perform the higher bending resistance have been desired. Thus, the above-described performance-weight efficiency must be a substantially important factor in developing the frame for vehicles.
Japanese Patent Laid-Open Publication No. 11-208521 discloses a structure in which a plurality of hollow spaces which extends in the longitudinal direction of the side sill is formed in a surrounding area including the corner portion of the inner panel of the side sill. This structure can perform the higher bending resistance compared with a conventional frame equipped with flat plates simply, like the above-described structure disclosed in the first publication. In this case, since the hollow spaces are formed, the entire weight of the frame can be lighter and the bending resistance of the frame can be improved.
Japanese Patent Laid-Open Publication No. 2008-68759 shows a mechanism of the bending deformation which has been verified by analyzing deformations of the frame in case the frame has the bending deformation. It also shows from the verification results that in case the load acts on the frame, a large outer-face deformation (herein, the outer-face deformation means a general deformation which comprises not only a two-dimensional deformation which occurs on the face but also a three-dimensional deformation which additionally includes a deformation occurring in the direction of thickness of the face) occurs at the corner portion and its surrounding area on which the compressive force acts. In a structure disclosed in this publication, the thickness of the above-described corner portion and its surrounding area is made thinner than any other parts from the above-described perspective, so that the weight reduction of the frame and the improvement of the bending resistance are achieved.
The conventional structures disclosed in the above-described publications, however, simply aimed at performing the higher bending resistance at the initial stage of the bending deformation of the frame for vehicles. In fact, there is a case in which it is preferable at some part of the vehicle body that the impact energy be absorbed more at the middle stage of the frame bending deformation in order to protect passengers properly against the vehicle collision or the like. Thus, in these days, it has been a problem to be solved that the performance-weight efficiency of the frame (the value of the amount of the impact energy absorption divided by the entire weight of the frame) in the impact energy absorption characteristics is increased as well as the impact-energy absorption at the middle stage of the frame bending deformation is improved.